Color concentrates are well known in the art. These blends which contain a relatively high concentration of a functional additive may be used advantageously in admixture with, for instance, virgin resin or with virgin blends in preparing molding compositions containing effective amounts of the functional additive.
Molders use color concentrates as means to minimizing inventory of pre-colored materials. Use of concentrates also provided for ease in handling, and maintaining cleanliness in work areas. For example, a molder requiring a large number of compositions differing in their color, may inventory lower volumes of each of these in the form of color concentrates and dilute them as needed with natural resins at the molding or extrusion machines to produce colored articles. Color concentrates containing 10 to 20% of the pigment are common in the industry. Titanium dioxide is a known pigment that is often useful in making white or opaque articles. Color concentrates containing polycarbonate resin and a high loading of titanium dioxide are therefore useful and desirable. Unfortunately, degradation of polycarbonate resin often results in the course of processing of such concentrates. There is a need in providing polycarbonate/titanium dioxide concentrates, which will offer greater resistance to thermal degradation. While the present inventors can not confirm it, a suggestion has been made that the degradation is a consequence of a reaction between the polycarbonate resin and impurities commonly present in commercial titanium dioxide pigments.
The present invention resides in the findings that polycarbonate color concentrates that contain a high loading of titanium dioxide are rendered more resistant to thermal degradation by the incorporation of a stabilizing amount of an additive, the structure of which contains cyclic iminoether groups.
The efficacy of oxazoline-containing additives in the context of polymeric resins, including polycarbonates, has been previously disclosed. Among the relevant documents, mention may be made of U.S. Pat. No. 4,895,897, which disclosed a polycarbonate containing thermoplastic molding composition having improved impact performance. The composition contains a functionalized elastomer which is the reaction product of (i) an ethylene elastomer having at least one reactive polar group and (ii) oxazoline-functionalized polystyrene. Surfaces with high gloss are said in U.S. Pat. No. 5,274,034 to characterize parts molded from a polycarbonate composition. This composition also contains aromatic polycarbonate with reactive carboxylic acid group and styrene-based polymer with repeating units containing pendant cyclic iminoether groups. The polymers and oligomers of olefinically unsaturated hydrocarbons containing oxazolinyl end groups have been disclosed in U.S. Pat. No. 5,756,621 for their utility as compatibilizers in polymer blends. Also relevant is U.S. Pat. No. 4,590,241, which disclosed a compatible blend of thermoplastic resins. The blend contains a reactive polymer having repeating units containing pendant cyclic iminoether groups. That polymer is reacted, to form a linkage and a compatible blend, with a thermoplastic polymer containing a co-reactive group.